Vitrification of in vivo produced equine embryos: Size really does matter!

Vitrification rather than slow-freezing has become the method of choice for the cryopreservation of in-vivo produced embryos. It is a simple technique that does not require a programmable freezing machine and relies chiefly on placing the embryo on a surface, rather than within a straw, in a small volume of vitrification solution to achieve extremely rapid cooling rates. However, in the field of equine embryo vitrification size truly does matter. Methodologies for both vitrification and warming prior to transfer are largely dictated by the size of the embryo. Since embryo size is directly related to age, the developmental changes that occur as an embryo develops have a direct impact on the protocols used for vitrifying and warming equine embryos. Embryos ≤450 μm offer the most practical option for vitrification and are associated with the highest success rates for practitioners wishing to vitrify and warm embryos. Larger embryos, however, require aspiration of the blastocele prior to vitrification, a procedure that necessitates technical expertise and the use of specialised equipment such as a micromanipulator. This review paper focuses on the vitrification of in vivo produced equine embryos and the advances made in vitrification and warming techniques aimed at improving practicality and pregnancy outcomes.